Phase position modulator



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PHASE POSITION MODULATOR Filed Aug. 10, 1961 2 Sheets-Sheet 2 FIG.4 14

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A 7' TORNE V United States Patent 3,138,665 PHASE PUSITION MODULATORNorman E. Chasek, Stamford, Conn., assignor to Bell TelephoneLaboratories, Incorporated, New York, N311, a corporation of New YorkFiled Aug. 10, 1961, Ser. No. 130,626 12 Claims. (Cl. 178-66) Thisinvention relates to communications systems and, more particularly, tophase position modulators for use in such systems.

A primary object of the invention is to simplify the construction ofphase position modulators, particularly of the type used for high speedoperation. A related object is to increase the versatility of phaseposition modulators such that modulation codes of both n-phase positionsand those comprising a combination of n-phase positions and n-amplitudepositions may be produced.

As is well known, phase position modulation, often referred to as phaseshift keying (PSK), has been employed for many years in the transmissionof slow speed telegraph signals over Wire. Microwave versions of thisform of modulation have also become known as polar data transmissionsystems. In such systems, the signal information is generally encodedinto a binary form wherein, for example, a one represents a pulse and azero represents a space, or the absence of a pulse, in accordance withconventional pulse code nomenclature. By convention, the pulses arenormally distinguished from the spaces in a phase position modulatedsignal by means of a carrier phase shift of, for example, 180 degrees.In another scheme, often referred to as differential phase shift keying,successive pulses shift the carrier phase back and forth between twopredetermined reference phases. A combination of binary phase positionand amplitude modulation'permits encoding to the base three or higherwith considerable precision. Primary advantages of phase positionmodulation over conventional on-oif AM modulation are that approximatelya 6 db noise advantage and over a 6 db saving in peak transmitter powerare realized with no increase in radio-frequency bandwidth. For thesereasons, phase position modulation is a strong competitor of othermodulation systems.

In the past, there have been various techniques employed for effectingthe necessary phase shifts in phase position modulators. For example, apair of amplifiers driven in phase, connected across a common outputtank circuit and alternately triggered has been utilized to effect 180degree phase shifts. Similarly, modified Wheatstone bridge circuits andcross-coupled diode switching matrices have also been employed to effecta desired phase shift of a radio-frequency carrier. Unfortunately, suchprior art forms of phase shift modulators do not afford the simplicityand high speed capabilities desired in a high frequency, large capacitycommunications system. In addition, the aforementioned types ofmodulators do not lend themselves readily to modifications, for example,to produce modulation codes not only of nphase positions but also ofn-amplitudes. Phase position modulators capable of operating other thanin a binary manner are, of course, very desirable because with them anincrease in data transmission capacity for a given bandwidth is obtainedwithout appreciable degradation in the signal to noise (8/ N) ratio.

In accordance with an aspect of the present invention, the disadvantagesand limitations of prior art phase position modulators are substantiallyalleviated by utilizing the negative resistance characteristics of atunnel diode in a unique manner to switch the phase of a radio-frequencycarrier between any given number of predetermined phase positions. Inone illustrative embodiment,

3,138,665 Patented June 23, 1964 binary modulation is accomplished bymeans of a tunnel diode (negative resistance) and a resistor (positiveresistance) connected in series across the terminals of a radiofrequencycarrier source. When a fast-acting switch, such as a diode,substantially reduces the magnitude of the positive resistance in thecircuit in response to alternate pulses, for example, from a PCMencoder, the radiofrequency carrier phase is shifted. If the positiveresistance is reduced to zero, as by placing a short-circuit connectionacross it, the carrier phase is shifted by degrees. Conversely,reinserting the positive resistance into the circuit effects a shift inphase of the carrier by 180 degrees.

In accordance with another aspect of the invention, the utilization of acombination of multivalued resistors, capacitors and inductorsselectively connected in series with the tunnel diode enables modulationcodes of n-amplitudes and n-phase positions to be produced.

The invention will be fully apprehended from the following detaileddescription of preferred illustrative embodiments thereof considered inconnection with the appended drawings, in which:

FIG. 1 is a schematic circuit diagram of a binary phase positionmodulator embodying the invention;

FIG. 2 is a diagram showing the current-voltage characteristic of atunnel diode and, in particular, the negative resistance region thereofutilized in the invention;

FIG. 3 shows several Waveforms of assistance in the exposition of theinvention;

FIG. 4 is a schematic circuit diagram of a phase-amplitude modulatorembodying the invention;

FIG. 5 is a schematic diagram of a three phase position modulator inaccordance with the invention;

FIG. 6 is a schematic diagram of a four phase position modulator inaccordance with the invention; and

FIG. 7 is a block diagram of a coherent carrier regenerator and detectorsuitable for use in demodulating three and four phase position signalinformation.

Referring now more particularly to the drawings, FIG. 1 depicts a binaryphase position modulator 10 comprising a radio-frequency carrier source11 having two output terminals 12 and 15. A resistor 14 indicatedbetween source 11 and terminal 12 represents the internal seriesresistance of the source 11 and the total lead resistance. A high speedswitch 18, shown symbolically within the dashed line enclosure, isconnected across a resistor 16 and is responsive to an appropriate formof modulating intelligence, such as pulse code information. The switch18 may comprise any one of a number of electronic responsive types; itis shown as a simple, two-pole electromechanical relay 19 actuated by acurrent source 20 in combination with an input pulse train 21. As willbecome more apparent hereinafter, a high speed switching diodeisparticularly applicable, for example, when it is desired to convertfrom an on-olf pulse form of modulation to a phase position form. Inthis case, the switching diode is simply biased to have two switchingstates, one representing an open or nonconducting state and the other a.closed or conducting state with respect to the flow of currenttherethrough. Such alternate states of bias are most easily accomplishedby utilizing a conventional flipflop circuit actuated by a triggercircuit responsive to the presence and absence of pulses, such asproduced by a PCM encoder. Numerous other ways to accomplish the sameresult are, of course, well known to those skilled in the art.

In accordance with one aspect of the invention, a negative resistanceelement, preferably comprising and shownas a tunnel diode 15, isconnected in series with an appropriate positive resistance element,e.g. resistor 16; and the series combination is connected in shunt withthe source 11.

For purposes of this invention, it is believed sufficient to state thatbecause of its unique characteristics, the tunnel diode offers manyadvantages over conventional prior art negative resistance devices, suchas the dynatron and point contact transistor in the common-emitterconfiguration. These include, in particular, extremely small negativetime constants and very pronounced and substantially linear negativeresistance regions. The latter characteristics in particular are madeuse of in this invention. For a detailed description of the solid statephysics of the tunnelling process which gives rise to the negativeresistance characteristic of these diodes, reference may be made to anarticle entitled New Phenomenon in Narrow Germanium P-N Junctions, L.Esaki, Physical Review, volume 109, pages 603604, 1958.

FIG. 2 depicts a typical current versus voltage characteristic of atunnel diode. A direct-current load line 31 and an alternating-currentload line 32 are shown mutually intersecting at a point 33 situated inthe intermediate negative resistance region of the I-V curve 30. As seenin FIG. 2, the negative resistance region is quite pronounced and existsbetween the peak of the curve designated point A and the valleydesignated point B. Waveform 34 shown relative to thealternating-current load line 32, depicts the constant current carrierwave generated by source 11, and waveform 35 shown relative to a linedrawn as a projection from the operating point 33 depicts thecorresponding portion of the carrier wave across the tunnel diode 15.

The unique function of the tunnel diode 15 in modulator will now beconsidered in greater detail (such function also being applicable to theother modulators to be described hereinafter). In accordance with theinvention, a unique relationship exists between the operating value oftunnel diode negative resistance and the positive resistance connectedin series with it. Specifically, these resistances are chosen such thatwhen the positive resistance is selectively short-circuited, the signonly of the resistance in shunt with source 11 changes; the resistancemagnitude remains substantially constant. It is this change in sign ofresistance which effects the de-. sired shift in phase of the appliedcarrier, such as by 180 degrees. Accordingly, with both resistanceelements in the circuit, the effective value of resistance isnecessarily of positive sign and results in no phase shift of an appliedcarrier. When the positive value of resistance is short-circuited orreduced to zero, however, the resistance magnitude in shunt with thesource still remains constant but the sign changes because of theremaining negative resistance of the tunnel diode. It is thus seen thatthe modulator may effect a carrier phase shift of 180 degrees every timethe positive resistance in series with the negative resistance is eithershort-circuited from or reinserted into the circuit, as in either casethe effective resistance in shunt with source 11 changes sign. Thenecessary value of positive resistance required to accomplish a 180degree phase shift in the modulator 10 is given by the following circuitequation:

where the subscripts correspond to the reference numerals of the circuitresistance elements.

That the phase of the radio-frequency carrier shifts 180 degreeswhenever resistor 16 is either short-circuited from or reinserted intothe circuit of modulator 10 may perhaps best be seen from the followingconsiderations. If the output of the carrier source 11 is defined as Isin w t and the net resistance across the terminals 12 and 13 is alwaysconstant, but selectively exhibits a positive or negative sign, itfollows that the voltage across the carrier source terminals may bedefined as o("1s"15) Sin e when resistor 16 is in the circuit (switch 18open) and I ["r 5l sin w t=I r SID. (M iji-77') when resistor 16 isshort-circuited (switch 18 closed).

As the value of negative resistance will of course depend on both thebias voltage applied to the tunnel diode and the current flowing throughit, the carrier source 11 is preferably of the constant current type.The neces sary bias for the tunnel diode 15 is supplied from adirectcurrent supply 23 through a low-pass filter 24. Filter 24comprises, for example, a capacitor 25 shunting the tunnel diode and anLC network including inductors 26, 27, and a capacitor 28 in shunt withthe supply 23. A variable resistor 29 in series with the supply isutilized to adjust the bias of the tunnel diode to the desired value. Byutilizing a carrier source of the constant current type, the capacitor25 and inductors 26, 27 of filter 24 also serve to restrict thefrequency of any possible self-oscillation of the tunnel diode 15 to thecarrier frequency f and, hence, such oscillation will prove to be of nopractical consequence.

With the components of modulator 10 thus set forth, its

' operation as a binary phase position modulator in accordi the carrierwave across the output terminals 12, 13 may arbitrarily be designated toexhibit a plus one (+1) for the zero or uniform phase condition, withswitch 18 open, and to exhibit a minus one (1) corresponding to thedegree out-of-phase condition whenever switch 18 short-circuits resistor16. The switching chart to the left of the modulator depicts theswitching sequence for this illustrative mode of operation.

To envisage pictorially how the original waveform generated by theconstant current carrier source 11 is phase position modulated,reference is made to FIGS. 3A and 313. FIG. 3A depicts the mark-spacesignal information, which may represent an analog signal sampled andquantized in code form, utilized to actuate selectively switch 13. Thesolid line curve 36 of FIG. 3B depicts the radiofrequency carrier waveof constant current and phase, eg, from source 11, and the dashed waveportions 37 depict the carrier wave shifted 180 degrees. The in-phase(solid line) condition is obtained with resistor 16 in the circuit andthe out-of-phase (dashed line) condition is obtained with resistor 16short-circuited from the circuit.

Considered more specifically, if it is assumed that switch 18 of FIG. 1is originally open, then the first pulse 38 of the code train depictedin FIG. 3A will cause the switch to close, thereby removing the positiveresistance (r from the circuit. This changes the sign of the resistancein shunt with the source 11 which, in turn, effects a carrier phaseshift of 180 degrees. The dashed line portion of waveform 37 shownimmediately below pulse 38 in FIG. 3B depicts this condition. Similarly,the absence of a pulse or, more specifically, a space between pulses,indicated as 39 in FIG. 3A, likewise will actuate switch 18 such that itopens. This reinserts resistor 16 into the circuit, changes the sign ofthe effective resistance across the source and thereby shifts thecarrier phase 180 degrees, i.e., back to its original phase conditionrepresented by the solid line waveform 36 in FIG. 3B. As may be seen bythe waveforms in FIGS. 3A and 3B, whenever two or more pulses occur insuccession, the switching action is responsive to the presence of onlythe first of the pulses in the succession. In other words, the switchingcircuit is actuated only when there is a change from a space to a pulseor a pulse to a space. This switching characteristic is also generallypreferred in the modulators to be described hereinafter.

FIG. 4 depicts in diagrammatic form a ternary phase position modulator40 embodying the principles of the invention. In addition to the circuitelements of modulator 40 which correspond to and are similarlyidentified as those of modulator 10, a second electronic switch 41 isemployed. Both switches 18 and 41 are shown symbolically within thedash-lined enclosed boxes as simple, single pole mechanical switches forpurposes of illustration. In practice, of course, these switches wouldbe of the high speed electronic type, such as switching diodes. As maybe seen in the chart to the left of modulator 40, a code to the basethree can easily be adopted in the form of a plus one, zero and minusone. For example, a plus one may arbitrarily be assigned to indicatethat both switches 18 and 41 are open, a zero to indicate that switch 41is closed (no carrier output) and, a minus one used to indicate thatswitch 18 only is closed. With the negative and positive resistancevalues in modulator 40 chosen as described above for modulator 10, it isevident that whenever switch 18 is closed or opened, the carrierWaveform is shifted 180 degrees, whereas when switch 41 is closed theradiofrequency carrier output across the terminals 12, 13 is reduced tozero. Switches 18 and 41 may be suitably actuated, for example, byconverting a conventional unipolar pulse train into a bipolar pulsetrain characterized by code symbols of positive, negative and zeroamplitudes. As such, switch 18 may be made responsive only to positivepulses and switch 41 may be made responsive only to negative pulses.Other encoding schemes to actuate these switches for ternary modulationapplications will be obvious to one skilled in the art.

An advantage of the ternary modulator 40 over the binary phase positionmodulator is that approximately a 60 percent decrease in bandwidth isrealized for the same signal transmission capacity. This is obtained,however, at the expense of approximately a 6 db decrease in thresholdvalue, i.e., the point at which the modulator cannot detect whether thesymbols of an input ternary code train identify accurately the originalcode conditions of zero, plus one or minus one amplitude.

FIG. 5 depicts in diagrammatic form a three phase position modulator 50embodying the principles of the invention. In addition to the circuitelements corresponding to those of modulator 40 and similarlyidentified, modulator 50 further includes a capacitor 51 and an inductor52, serially connected with the tunnel diode 15 and the resistor 16 inshunt with source 11. This arrangement permits the transformation ofsignal information, for example, from a three-level unipolar or abipolar encoder, into a three phase position code wherein carrieramplitude remains substantially constant and only carrier phase isselectively changed. For example, the values of circuit resistance andreactance may be so chosen that the train of input pulses will actuatethe switches 18 and 53 in a manner to establish the carrier at any oneof three symmetrically oriented phase positions 120 degrees apart. Toobtain such phase shifts at 60, 180 and 300 degrees with the switchingsequence illustrated in the chart associated with FIG. 5, for example,the impedances of the reactive circuit elements can be derived from thefollowing equations:

and

where X and X represent the inductive and capacitive reactances ofmodulator 50, respectively. A virtue of modulator 50 in a completecommunications system is that it has approximately a 1 db net advantage,in terms of threshold, over the binary phase position modulator of FIG.1, and about a 5 db net advantage over the ternary modulator of FIG. 4.This follows from the fact that in a three phase position system a peaksignal to peak noise voltage ratio of approximately 1.15 is required tointroduce errors into the system at the receiver. This can be shown tobe about 1.2 db poorer than the value of threshold for a binary phaseposition modulator. However, for the same information rate, thebandwidth is reduced by about 60 percent as in the case with the ternarymodulator of FIG. 4. As this reduc tion in bandwidth results in a 2 plusdb advantage over the binary system, the net advantage of the threephase position system is seen to be approximately 1 db.

FIG. 6 depicts in diagrammatic form a four phase position modulator 60embodying the principles of the invention. The basic structuraldilference of modulator 60 from modulator 50 is the connection of switch18 across only the positive resistor 16. This change is required so thatthe respective switches may be made selectively and independentlyresponsive to different code symbols of the input signal information. Inother words, switch 18 when closed cannot over-ride switch 53 as inmodulator 50 since the carrier, in the four phase position case, isperiodically modulated in phase quadrature with the 0-180 degree phasepositions. By suitably choosing the values of resistance and reactanceacross the phase shift network, phase shifts of 45, 135, 215 and 315degrees, for example, can easily be obtained by selectively actuatingswitches 18 and 53. The chart associated with modulator 60 in FIG. 6depicts one illustrative switching sequence for producing a four phaseposition modulated signal when the circuit parameters are chosen toeffect a 45 degree reference phase position with both switches 18 and 53open. Other than in requiring an input pulse code train characterized byfour rather than three discrete code symbols to actuate the switches,the operation of modulator 60 is essentially identical to that ofmodulator 50 described above.

From the foregoing description of the illustrative embodiments, itbecomes apparent that modulation codes of n-phase positions as well asn-amplitudes can be readily obtained. For example, additional switchesmay be used to add or remove positive resistors, capacitors, andinductors from the circuit. These elements may be connected as required,in series or parallel (or in combinations of series and parallel), withthe negative resistance device 15. Moreover, while the variousmodulators have been described in terms of operation with aradio-frequency carrier of constant amplitude as shown in FIG. 3B, it isto be understood that the carrier may be converted to a raised cosinewaveform by well known techniques. As such, the desired phase shifts ofthe carrier wave may be effected at times when the amplitude of thecarrier wave is reduced to zero. This may be advantageous in certainapplications to prevent or minimize any undesired transient effects thatcould possibly occur with each phase reversal of the carrier.

While conventional binary or phase shift keying receiver apparatus maybe used to demodulate binary encoded'in formation, more refinedapparatus is required to demodulate information coded to the base threeor higher. For example, unique apparatus must be provided to preventwobble in the reference phase of the regenerated carrier caused byvarying phase quadrature components. In addition, detector apparatus isrequired in the three phase position case to ascertain in which of thephase sectors the signal vector is located with respect to the referencesector so that the original signal information may be reconstructed forutilization.

In my copending application, Serial No. 130,610, filed concurrently withthis application, unique carrier regenerator and detector apparatus aredisclosed for the demodulation of both binary and higher base phaseposition codes.

By way of illustration, FIG. 7 depicts, in block diagram form, a circuit70 for the demodulation of three or four phase position coded signalinformation comprising a coherent carrier regenerator network '72 and aphase detector 74 each of the type described in my aforementionedcopending application. The detector may include a phase vector sensorand a pulse regenerator. The incoming three or four phase positionsignal information received from a modulator 73, which may be of thetype depicted in either FIG. or 6, described above, depending on thenumber of phase positions, is applied to carrier rcgenerator network 72and, more specifically, to a phase shift network 75 and a lockedoscillator 76. The network 75 includes a phase shift circuit similar tothat employed in the particular one of modulators 50 and 60 employed atthe transmitting end of the system. The periodic phase differences thatoccur between the reference phase at the output of the locked oscillator76 and the phase of the received signal are detected and transformedinto two distinct control pulses. These control pulses in turn actuatesuitable switching circuits in the phase shift network 75 in order toremove from or restore to the phase shift network predetermined valuesof resistance and reactanee in much the same manner as described abovefor the three and four phase position modulators. Such switching actionetfects selective, predetermined in-phase and quadrature phase shiftinversions of the receiving signal which, in turn, produces atimevarying resistance function exhibiting frequency componentspeculiarly related to the signal and carrier frequencies. Specifically,the product of the frequency components established by the time-varyingresistance function and the sideband frequency components of thereceived signal produce a strong coherent carrier of the proper phasewhich keeps locked oscillator 76 in precise phase and frequency with theoriginal carrier.

In the four phase posiiton case, the in-phase and quad- 77 of the lockedoscillator are separately applied to conventional homodyne detectors fordemodulating the signal information in much the same way as for ordinarybinary signals. A portion of the regenerated carrier must be shifted 90degrees, however, to demodulate the phase quadrature components of thereceived signal information.

In the three phase position case, the output of the locked oscillator isapplied to the detector 74. This circuit is designed to ascertain inwhich of the phase sectors the signal vector (respresentative of anoriginal pulse or space) is located with respect to the reference sectorso that the original signal information may be accurately reconstructedfor utilization. A more detailed description of the phase positiondemodulator of FIG. 7 and the manner in which it functions may be foundin my aforementioned copending application.

It is to be understood that the specific embodiments described hereinare merely illustrative of the general principles of the instantinvention. For example, the switching means utilized in the modulatorcircuits may be made responsive to a train of input pulses in a mannerwhereby only successive pulses shift the carrier a predetermined numberof degrees, the spaces causing no phase shift in the modulation process.In such a system, the decision process at the receiver depends upon thepreviously transmitted signal for the phase reference, resulting inadjacent bit dependence with respect to error probability. Detractingfrom the advantages of this form of phase position modulation, however,is the tendency rature components of the received signal at the outputfor errors due to noise to occur in pairs. Numerous other structuralarrangements and modifications similarly may be devised in the light ofthis disclosure by those skilled in the art without departing from thespirit and scope of the invention.

What is claimed is:

1. A modulator for converting pulse encoded information into phaseposition encoded information comprising a negative resistance elementand a positive resistance element connected in series across theterminals of a source of a radio-frequency carrier, and means responsiveto a given code character of said pulse code information connectedacross one of said resistance elements for selectively altering theresistance thereof such that the phase of said carrier is shifted fromone predetermined phase condition to a second predetermined phasecondition.

2. A modulator in accordance with claim 1 wherein said negativeresistance element comprises a tunnel diode and wherein said meansconnected across one of said resistance elements comprises a signalresponsive switching element connected across said positive resistanceelement.

3. A modulator in accordance with claim 1 further comprising switchingmeans positioned across the terminals of said carrier source and meansfor selectively operating said last-mentioned switching means inresponse to a different code character of said pulse code informationfor providing a ternary phase position coded signal modulated output.

4. A modulator for converting signal information previously encoded inpulse code form into a phase-position carrier wave form wherein theoriginal pulses and spacing intervals of said code are respectivelycharacterized by at least first and second predetermined time-phasepositions of the carrier wave, comprising a source of a radiofrequencycarrier means for shifting the phase of said carrier wave selectivelybetween said first and second predetermined phase positions, said meansincluding a twoterminal semiconductor element biased in its negativeresistance region and a positive resistance element connected in seriesacross the terminals of said source, a first high speed switch connectedacross said positive resistance element responsive to a firstidentifiable code character of said pulse code information, the presenceof said first code character being suflicient to close said switch,thereby to alter the magnitude of said positive resistance element.

5. A modulator according to claim 4 which includes means for shiftingthe phase of said carrier wave selectively by a predetermined number ofdegrees from one of said first and second phase positions, said meansincluding an inductor and a capacitor serially connected with saidtwo-terminal semiconductor element and said positive resistance elementacross the terminals of said source, a second high speed switchconnected across said inductor responsive to a second identifiable codecharacter of said pulse code information, the presence of said secondcode character being sufficient to close said second switch thereby toalter the inductance of said inductor.

6. A modulator in accordance with claim 5 which includes a third highspeed switch responsive to externally applied code signals connectedacross the series combination of said positive resistance element, saidcapacitor, and said inductor, the presence of a code signal beingsufiicient to close said third switch thereby to alter the magnitude ofthe resistance and reactanee of said series combination.

7. Apparatus for'converting signal information previously encoded inpulse-code form into phase-position form comprising a two-terminalnegative resistance semiconductor element,--a positive resistanceelement, an inductor, and a capacitor connected in series across theterminals of a source of a radio-frequency carrier, first switchingmeans connected across the series combination of said positiveresistance element, said capacitor, and

said inductor responsive to identifiable code characters of said pulsecode signal information, second switching means connected across saidinductor only responsive to different identifiable code characters ofsaid pulse code signal information, and means for selectively actuatingsaid first and said second switching means whereby the sign of the totalresistance shunting said source is reversed and the magnitude of theinductance of said inductor is altered thereby to shift the phase ofsaid carrier wave successively between at least three distinctpredetermined phase positions.

8. Apparatus for shifting the phase of a radio-frequency carrier fromone predetermined phase position to a second comprising a source ofcarrier signals, a negative resistance element which exhibits apreselected absolute value of resistance, a positive resistance elementwhich exhibits substantially the same preselected absolute value ofresistance connected in series across the terminals of said carriersource, and means responsive to a first code signal for altering theresistance of one of said resistance elements whereby the magnitude ofresistance remaining in shunt with said source is substantially constantbut the sign thereof is changed.

9. Apparatus in accordance with claim 8 wherein said negative resistanceelement comprises a tunnel diode biased in its negative resistanceregion and wherein said means for altering the resistance of one of saidresistance elements comprises switching means responsive to said codesignals for altering the resistance of said positive resistance element.

10. Apparatus in accordance with claim 8 further comprising a capacitorand an inductor serially connected with said negative and positiveresistance elements in shunt with said source, second means responsiveto a second code signal for altering the reactance of said seriescombination of said inductor and said capacitor thereby to shift thephase of said carrier a predetermined number 10 of degrees with respectto said first and said second predetermined phase conditions.

11. Apparatus in accordance with claim 10 wherein said first and saidsecond means for altering the resistance and reactance respectively ofsaid serially connected impedances comprise first and second two-statesignal actuated switches, said first switch being connected across thepositive resistance element and said second switch being connectedacross said inductor.

12. A modulator for converting signal information previously encoded inpulse code modulation form into phase position form comprising in seriesrelation a tunnel diode, a positive resistance element, an inductor anda capacitor connected in shunt across the terminals of a radio-frequencycarrier source, means for biasing said tunnel diode in its negativeresistance region, first means for selectively short-circuiting saidpositive resistance element, capacitor and inductor in response to afirst identifiable character of said pulse information thereby to shiftthe phase of said carrier Wave from a first predetermined phasecondition to a second predetermined phase condition and second means forselectively short-circuiting the inductance of said inductor in responseto a second identifiable character of said pulse information thereby toshift the phase of said signal information selectively a predeterminednumber of degrees from one of said first and second phase conditions ofsaid carrier Wave, respectively, to at least a third predetermined phasecondition.

IBM Technical Disclosure Bulletin, vol. 3, No. 5, Bistable Memory,October 1960.

1. A MODULATOR FOR CONVERTING PULSE ENCODED INFORMATION INTO PHASEPOSITION ENCODED INFORMATION COMPRISING A NEGATIVE RESISTANCE ELEMENTAND A POSITIVE RESISTANCE ELEMENT CONNECTED IN SERIES ACROSS THETERMINALS OF A SOURCE OF A RADIO-FREQUENCY CARRIER, AND MEANS RESPONSIVETO A GIVEN CODE CHARACTER OF SAID PULSE CODE INFORMATION CONNECTEDACROSS ONE OF SAID RESISTANCE ELEMENTS FOR SELECTIVELY ALTERING THERESISTANCE THEREOF SUCH THAT THE PHASE OF SAID CARRIER IS SHIFTED FROMONE PREDETERMINED PHASE CONDITION TO A SECOND PREDETERMINED PHASECONDITION.